How a diversity of mortality mechanisms are countered by a diversity of survival systems in forest communities

Sean M McMahon, Forest Global Earth Observatory, Smithsonian Environmental Research Center, Edgewater, MD, Daniel F. Zuleta, ForestGEO and NGEE–Tropics, Smithsonian National Museum of Natural History, Washington, DC, Gabriel Arellano, University of Michigan, Ann Arbor, MI and Stuart J. Davies, Forest Global Earth Observatory, Smithsonian Tropical Research Institute, Washington, DC

Background/Question/Methods
Global change is altering the frequency and intensity of biotic and abiotic stressors on forest communities. There have been several striking examples of mortality increases due to climate and pest outbreak events, but there is equivocal evidence of baseline changes in mortality rates across temperate and tropical forests. This may be due to several factors: uncertainty in the actual conditions species are confronting, uncertainty in our ability to record responses, but perhaps more overlooked are the abilities of species to tolerate or resist stressful conditions. We will explore this third knowledge gap with a conceptual look at expectations of mortality response across species. We will also investigate the potential role of senescence in inference about mortality patterns, physiological resistance to damage, recovery from damage, and acquired resistance to pathogens.

Results/Conclusions
We use an annual mortality survey to show how mechanisms that may increase mortality are distributed across diverse communities, with multiple mechanisms or no mechanisms responsible for the majority of observed deaths. We then review examples from the literature of the systems tree species rely on to survive the many mortality risks they are exposed to in order to achieve long lives. We conclude with a simple demographic model of how these survival systems might allow different species to navigate changes and persist, as well as indicate potential threats to populations when these systems fail. Our goal is to add some nuance to analyses of forest mortality research given the diversity of life history strategies that are designed for exposure to many decades of threats.